1. Field of the Invention
The present invention relates to dental impression compositions based on silicones which crosslink by an addition reaction. The impression compositions according to the invention are distinguished by exhibiting a moderate viscosity, determined in accordance with ISO 4823, in the mixed state, good hydrophilicity and good flow capability simultaneously with outstanding stability. The recipes according to the invention are particularly suitable for mechanical mixing in automatic mixing apparatuses (e.g. Pentamix.RTM., ESPE, Seefeld). The materials are employed as so-called monophase impression materials and, because of their flow properties and their ease of cutting, can be used in the practice as a substitute for alginates.
2. Description of the Conventional Art
In the dental practice, impression materials based on alginates are often employed for taking situation impressions of teeth and parts of the jaw for evaluation, diagnosis, planning and monitoring the accuracy of the fit of conservation, prosthetic and orthodontic work. In this procedure, the rough form of the jaw and teeth is recorded instantaneously by preparing a so-called situation impression. After the situation impression has been filled with gypsum suspension, the dentist then has the so-called study model, diagnostic model, documentation model or working and planning model. So-called counter-jaw models are also made to reproduce the opposite jaw for more extensive prosthetic work, and these are obtained by impressions with alginates.
An important field of use for alginate impression materials is the production of provisional crowns and bridges. In this procedure, a situation impression which records the starting situation is taken on the patient before the start of the preparation of one or more teeth. After the preparation has taken place, a provisional crown and bridge material, which is initially still in the pasty state, is incorporated at the appropriate points in the situation impression. This impression filled with the pasty material is then reset in the patient's mouth, where the crown and bridge material cures to form the provisional prosthesis. It is of particular importance here that the alginate material is easy to cut in the cured state, to influence shaping of the provisional prosthesis advantageously in favour of the stability by appropriate cutting of the alginate impression.
Alginates consist of derivatives of alginic acid which, after stirring with water, cure in a sol-gel process. The impressions are made as so-called one-phase or monophase impressions, i.e. only a single viscosity of the impression material is employed when taking the impression (see J. Wirz et al., "Abformung in der zahnartzlichen Praxis [Impressions in the dental practice]", 1993, Gustav Fischer Verlag, p. 7). Some of the essential advantages of alginates for the fields of use described are:
high hydrophilicity PA1 one-phase materials PA1 good flow-on of the paste into undercuts PA1 easy cutting of the cured impression PA1 easy removal of the cured impression from the mouth PA1 low costs. PA1 good ability to stand, i.e. no running of the paste off the impression spoon PA1 short setting time, i.e. short residence time of the impression composition in the mouth. PA1 no dimensional and storage stability of the cured impressions due to drying out. Because of this water loss and the resulting shrinkage, alginate impressions must be filled with gypsum suspension immediately after they are removed from the mouth. PA1 no filling with gypsum several times is possible. Only one gypsum model can be produced from each impression. For example, it the model breaks, it is necessary to take the impression again. PA1 disinfectability presents problems because of the tendency to absorb water and to swell. PA1 in some instances incompatibility with gypsum, so that rough surfaces can occur on the gypsum model. PA1 mixing as a rule is still done manually. Although some automatic mixing systems exist, they are of only minor importance, and furthermore also do not allow automatic metering into the spoon. As a result of the manual mixing, there are often air bubbles in the impressions, which lead to inaccuracies on the models. PA1 (a) organopolysiloxanes with at least two unsaturated groups in the molecule, PA1 (b) organohydrogenpolysiloxanes with at least 2 SiH groups in the molecule, PA1 (c) if appropriate organopolysiloxanes without reactive groups, PA1 (d) platinum catalyst, PA1 (e) hydrophilizing agent, PA1 (f) diatomaceous earth, PA1 (g) filler, and PA1 (h) if appropriate further customary additives, auxiliaries and dyestuffs,
Further advantages of alginate materials are
However, the use of alginates is also associated with some decisive disadvantages:
Silicones which crosslink by an addition reaction represent another class of dental impression materials and are currently used as precision impression materials to produce extremely precise working models for production of dental prostheses. The properties of such compositions are described e.g. in the standards ISO 4823 and ADA 19. Silicones which crosslink by an addition reaction are described, for example, in U.S. Pat. No. 4,035,453. Since their curing takes place by a platinum-catalysed addition mechanism, which does not function on an aqueous basis, in principle they do not show the phenomenon of dimensional instability due to water loss. Furthermore, impressions made of silicones which crosslink by an addition reaction can be filled with gypsum suspension as often as desired, so that several models with a smooth gypsum surface can be cast with a single impression. The disinfectability of the impressions is also non-problematic with silicones which crosslink by an addition reaction.
Silicone impression compositions are entirely hydrophobic and show the problem that the recording sharpness of the impression is unsatisfactory due to poor flow-on properties of the paste as a result of inadequate hydrophilicity. These problems are described e.g. in DE-A-38 38 587, page 2, lines 19-23 or in EP-A-0 480 238, page 2, lines 1-26.
Various additives which increase the hydrophilicity of silicone impression compositions have been described in the literature to solve this problem. An overview of the prior art is to be found, for example in EP-A-0 480 238, page 2, lines 20-38. Additives which have proved to be particularly effective are polyether-siloxanes, such as are described, for example, in the international application WO 87/03001 or in EP-B-0 231 420. Further very effective additives are so-called polyether-carbosilanes, such as are described in WO 96/08230. Ethoxylated fatty alcohol derivatives, see e.g. EP-B-0 480 238, are also suitable for increasing the hydrophilicity and therefore for improving the flow-on properties. By employing these surfactants, the flow-on properties of the pastes are improved considerably, so that a good wettability can be achieved.
Commercially available silicone impression compositions which crosslink by an addition reaction are usually in a two-component form and comprise a so-called base paste and catalyst paste, in which the reactive components are separated from one another spatially for stability reasons. Curing of the materials takes place after the two pastes have been mixed in precisely defined volume ratios. Mixing is as a rule carried out manually or by pressing out of double-chamber cartridges, the pastes being conveyed through a mixing tube containing a static mixer, as a result of which intimate mixing of the pastes occurs. However, only relatively small amounts of paste can be mixed with this in a short time.
A further development of the handling properties of silicone impression compositions which crosslink by an addition reaction consists of the development of automatic mixing and metering systems for two-component impression compositions which have automatic conveying and mixing units, such as are described e.g. in U.S. Pat. Nos. 5,249,862, 5,286,105 and 5,332,122. The need for manual mixing of base pastes and catalyst pastes, above all when mixing larger quantities of material, is thus eliminated, since this can take place automatically and within a short time. The result is a completely homogeneous product which is free from bubbles (see the brochure ESPE Pentamix.RTM., ESPE, Seefeld).
Silicones which crosslink by an addition reaction are chiefly used in a two-phase form: readily flowing or moderately flowing compositions are combined with poorly flowing or kneading compositions (see J. Wirz et al., p. 7).
There are indeed hydrophilic silicones which crosslink by an addition reaction and are employed in a monophase technique, i.e. are used in only a single viscosity. The use of such silicone impression compositions for the production of provisional bridges and crowns has not hitherto been thought of, since these materials are employed for taking precision impressions, which differs fundamentally from use for taking situation impressions. Commercially available hydrophilic monophase A silicones thus have quite a slow setting process, which as a rule is over 3 minutes for a setting time in the mouth (see J. Wirz, "Abformung in der zahnartzlichen Praxis [Impressions in the dental practice]", 1993, Gustav Fischer Verlag, p. 16 to 26). The setting time in the mouth is the time between positioning of the spoon with the impression material in the mouth of the patient and removal of the cured impression, and can also be called the mouth residence time or period. This is unfavourable for a situation impression material, since the dentist wants to save as much time as possible when working with this type of impression. Setting times of &lt;3 minutes mouth residence time, preferably &lt;2.5 minutes, and particularly preferably &lt;2 minutes, are desirable properties for the dentist here. For example, the hydrophilic one-phase impression material "Imprint 2:5" (3M) has a setting time of 5 minutes, while a typical alginate impression material ("Alginoplast", Bayer) has a setting time of 1.5 minutes.
Furthermore, hydrophilic monophase A silicones are expensive impression compositions and as a rule have quite high hardnesses in the cured state, so that their Shore hardness A--measured in accordance with DIN 53505, 30 minutes after the start of mixing of the pastes--is as a rule I&gt;50. Such impression compositions can therefore be separated from the objects of which the impression has been taken only with relative difficulty because of their hard nature. The crown and bridge material cured in the patient's mouth to give the provisional prosthesis is likewise difficult to separate from the impression material if this has too high a hardness. It could indeed be considered to modify the known hydrophilic monophase silicone impression materials by additives such that they are softer and more flexible in the cured state, so that they can then be separated easily from the objects of which the impression has been taken. An addition of plasticizers, such as e.g. commercially available plasticizers for plastics or silicone oils, also leads to a reduction in the costs. However, such plasticized silicone impression materials are then unacceptable in their ease of cutting. When producing the provisional crowns and bridges, the dentist must cut the cured impression at the interdental points, so that the material used for production of the provisional prostheses, which in itself is relatively brittle, does not become too thin at the interdental points and break. Experiments with hydrophilic monophase silicone impression materials which had merely been plasticized and had a Shore hardness A of less than 50 after curing could no longer be cut in the necessary way, since the interdental septa evade a scalpel elastically in the usual cutting technique.